The present invention relates to a traffic channel selecting method and base station equipment preferable for constructing a mobile communications system including a plurality of radio base stations and mobile stations.
More particularly, the present invention relates to a traffic channel selecting method and base station equipment applicable to such systems as a mobile communications system including a plurality of radio base stations and a plurality of mobile stations that carry out communications with them, and in particular to a system whose service area consists of a plurality of radio base stations assuming various cell radii, or to a system in which a plurality of systems, whose mobile stations that receive their services have different characteristics, share the same frequency band.
A mobile communications system such as mobile phones or car phones which are widely available today carries out its services by dividing its service area into comparatively small radio zones referred to as cells. Such a system comprises as shown in FIG. 1 a plurality of radio base stations 111 that cover the divided radio zones, and a plurality of mobile stations 112 that conduct communications with the base stations 111 by establishing radio channels.
Radio waves transmitted from the base stations 111 or mobile stations 112 at certain transmission power levels propagate through the space with their powers attenuated, and arrive at their destination terminals. The attenuation the ratio waves undergo increases with the distance between the transmitting terminals and the receiving terminals.
On the other hand, the receiving terminals require a specific level or over of the received power to achieve demodulation of the received radio wave at a desired quality.
Accordingly, increasing cell radii to extend the cover areas of the base stations requires to install in the base stations and mobile stations transmission equipment with greater power. In contrast, reducing the cell radii to decrease the cell areas can reduce the transmission power required by the base stations and mobile stations.
In the mobile communications, a processing what is called handover is carried out to continue communications by successively switching the radio base stations with the roaming of a mobile station. The handover processing, when considered from the standpoint of the cell radius, presents the following problem as the cell radius reduces. That is, the frequency of the handover processing rises, which in turn increases the processing load of the network. When instantaneous interruption takes place because of the handover, it has great influence on the conversation quality a user feels. It is also possible that the handover processing cannot catch up with the roaming of the mobile station, so that the conversation can be disconnected.
It will be understood from the description above that small radius cells are suitable for terminal equipment with low transmission power carried by pedestrians, whereas large radius cells are appropriate for terminal equipment with comparatively large power carried on motor vehicles. Thus, the small and large radius cells have their own advantages and disadvantages. Generally, a comparatively large cell with a radius from one kilometer to a few kilometers is often called a macrocell, and a rather small cell with a radius of a few hundred meters is commonly called a microcell.
Incidentally, users are not uniformly distributed geographically, but concentrate to limited places or roads. In view of this, such a method is taken that the macrocells roughly cover the area, and the microcells cover the places and roads in the area where the users concentrate.
Such a method that forms the area with the macrocells in combination with microcells is referred to as a micro/macro overlay system in the present specification.
The micro/macro overlay system is characterized in its establishing method of the radio channels. In the macrocells, the network conventionally carries out centralized management of the entire channels employed by the base stations.
In contrast with this, since the microcells are located in various propagation environments such as outdoors, underground shopping center and buildings in front of a station or in bustling streets as well as in homes, it is very difficult to assign radio channels considering the interference between the microcells. Since the systems in homes or offices operate as individual systems, it is difficult to apply to them a centralized management style assignment method. Thus, a method is desired in which the individual base stations establish their radio channels through their own autonomous control.
Conventionally, as a method for making a decision as to whether a particular channel is available or not, a radio base station or mobile station measures the received power of interference signals, and decides that the channel is available when the received power (interference power) is less than a fixed level.
However, applying the conventional method to the micro/macro overlay system, in which the microcells and macrocells share a common radio frequency band, has a great problem of being unable to avoid the interference between them.
FIG. 2 shows an example of making a decision as to whether a radio channel is available in a microcell adjacent to a macrocell. In FIG. 2, the horizontal axis represents distance, and the vertical axis represents transmission power or received power. Base stations A and C are low power base stations forming a microcell each. The base station A transmits at transmission power Pa through a radio channel f1. The base station B is a high power base station forming a macrocell, and transmits at transmission power Pb using a radio channel f2.
In FIG. 2, the base station C is newly installed in addition to the base stations A and B, assuming that the base station C autonomously determines the radio channel of its own use. As shown in FIG. 2, when the base station C receives signals from both the base stations A and B, the received levels of the signals at the base station C are identical at level L. If the level L is lower than the threshold value THR, both the frequencies f1 and f2 are available.
Let us consider here the interference given to the base station C from the mobile stations a and b that carry out communications with the base stations A and B, respectively. When the base station C selects the radio channel f1, it receives the interference from the mobile station a with low transmission power, which does not presents any problem. However, when the base station C selects the radio channel f2, it will receive the interference from the mobile station b with high transmission power. Thus, the base station C cannot use the radio channel f2 in practice.
Thus, in the conventional decision making as to whether the channel is available, the interference between the microcell and macrocell is unavoidable.
In view of this, therefore, a first object of the present invention is to propose a traffic channel selecting method and station equipment capable of avoiding in advance the interference from a mobile station transmitting at large power by making a decision as to whether the radio base station causing the interference is present in a macrocell or microcell.
A second object of the present invention is to propose a traffic channel selecting method and base station equipment capable of avoiding the occurrence of the interference, and constructing a mobile communications system with high quality and high frequency usage efficiency in a system whose service area consists of a plurality of radio base stations assuming various cell radii, or in a system in which a plurality of systems, whose mobile stations that receive their services have different characteristics, share the same frequency band.
In the first aspect of the present invention, there is provided a traffic channel selecting method in a mobile communications system including a plurality of radio base stations, comprising the steps of:
referring, in the particular radio base station, to transmission power read from a channel transmitted from each of remaining radio base stations;
correcting the threshold value such that the threshold value is reduced when the transmission power is large, and that the threshold value is increased when the transmission power is small; and
making a decision, by comparing a received level measured in the channel with the threshold value corrected, that the channel is available if the received level measured is lower than the threshold value, whereby setting a threshold value of a received level used for deciding availability of a traffic channel in a particular radio base station.
In the second aspect of the present invention, there is provided a traffic channel selecting method in a mobile communications system including a plurality of radio base stations, comprising the steps of:
referring, in the particular radio base station, to transmission power read from a channel transmitted from each of remaining radio base stations;
calculating, in the particular radio base station, a propagation loss between the particular radio base station and each of remaining radio base stations; and
making a decision that the channel is available, if a received level acquired by receiving the channel transmitted from each of the remaining radio base stations is less than a predetermined received level, and the propagation loss of the channel is less than a predetermined threshold value of the propagation loss, whereby making a decision on availability of a traffic channel in a particular radio base station.
In the third aspect of the present invention, there is provided base station equipment installed in a particular radio base station in a mobile communications system including a plurality of radio base stations, the base station equipment comprising:
means for measuring a received level of a channel transmitted from each of remaining radio base stations;
means for reading a transmission power value of the channel from each of the remaining radio base stations; and
means for making a decision on availability of the channel by the particular radio base station from the received level measured and the transmission power value read.
Here, the base station equipment may further comprise:
means for storing a threshold value of the received level in correspondence with the received level of the channel transmitted from each of the remaining radio base stations;
means for correcting, in accordance with the transmission power value of the channel from each of the remaining radio base stations, the threshold value of the received level stored; and
means for making a decision on the availability of the channel in the particular radio base station by comparing the received level with the threshold value corrected.
The base station equipment may further comprise:
means for storing a first threshold value corresponding to the received level of the channel transmitted from each of the remaining radio base stations;
means for calculating a propagation loss between the particular radio base station and each of the remaining radio base stations from the received level and the transmission power value of the channel from each of the remaining radio base stations;
means for storing a second threshold value in correspondence with the propagation loss calculated;
means for comparing the first threshold value with the received level;
means for comparing the second threshold value with the propagation loss; and
means for making a decision on availability of the channel in the particular radio base station from a result of comparing the first threshold value with the received level, and a result of comparing the second threshold value with the propagation loss.